KeY-ABS Tutorial

KeY-ABS is an interactive (semi-automatic) deductive verification tool that enables one to verify functional properties for concurrent and distributed ABS models with unbounded size. In this tutorial we demonstrate in detail how to specify and verify ABS applications. We strongly encourage reading this tutorial next to a computer with a running KeY-ABS system.

1 Introduction

KeY-ABS is a variant of the verification system KeY for sequential Java/JavaCard. It is based on the KeY 2.0 platform – a verification system for Java. We generalize various subsystems of KeY and abstract away the Java specifics. After refactoring the KeY system provides core subsystems (rule engine, proof construction, search strategies, specification language, proof management etc.) that are independent of the specific program logic or target language. These are then extended and adapted by the ABS and Java backends.

KeY-ABS is designed for the verification of concurrent and distributed ABS models. By following this tutorial, readers should be able to execute KeY-ABS on their own machine, reproduce the proof of the example, and understand what KeY-ABS can do. Sufficient background knowledge and references are provided so the readers can understand the technicality of how the tool works.

2 Tool Installation

For a local installation of the KeY-ABS theorem prover, install Java 8. Then, download KeY-ABS from http://heim.ifi.uio.no/~crystald/key-abs.zip. Unzipping the downloaded file and double-clicking on the key.jar file should start KeY-ABS. To start it from the command line, enter the directory key-abs and use:

java-jarkey.jar

3 System Workflow

The input files to KeY-ABS comprise (i) an .abs file containing ABS program and (ii) a .key file containing class invariants, functions, predicates and problem specific rules required for this particular verification case. Given these input files, KeY-ABS opens a proof obligation selection dialogue that lets one choose a target method implementation. From the selection the proof obligation generator creates an ABSDL formula, which will be explained in Section 4. By clicking on the Start icon the verifier will try to automatically prove the generated formula. A positive outcome shows that the target method preserves the specified class invariants. In the case that a subgoal cannot be proved automatically, the user is able to interact with the verifier to choose proof strategies and proof rules manually. The reason for a formula to be unprovable might be that the target method implementation does not preserve one of the class invariants, that the specified invariants are too weak/too strong or that additional proof rules are required. The workflow of KeY-ABS is illustrated in Figure 1.

Figure 1: Verification workflow of KeY-ABS

4 ABS Dynamic Logic

Specification and verification of ABS models is done in the ABS dynamic logic (ABSDL). ABSDL is a typed first-order logic plus a box modality: For a sequence of executable ABS statements S and ABSDL formulae P and Q, the formula P→[S]Q expresses: If the execution of S starts in a state where the assertion P holds and the program terminates, then the assertion Q holds in the final state. Given an ABS method m with body mb and a class invariant I, the ABSDL formula I→[mb]I expresses that the method m preserves the class invariant. In sequent notation P →[S]Qis written

where Γ and Δ stand for (possibly empty) sets of side formulae. A sequent calculus as realized in ABSDL essentially constitutes a symbolic interpreter for ABS. For example, the assignment rule for local program variables is

where v is a local program variable and e is a pure (side effect-free) expression. This rule rewrites the formula by moving the assignment from the program into a so-called update [1], as {v := e} shown above, which captures state changes. The symbolic execution continues with the remaining program rest. Updates can be viewed as explicit substitutions that accumulate in front of the modality during symbolic program execution. Updates can only be applied to formulae or terms. Once the program to be verified has been completely executed and the modality is empty, the accumulated updates are applied to the formula after the modality, resulting in a pure first-order formula. This is shown by the following rule emptybox.

The rule for the conditional splits the proof into two branches; one for the case where its guard b evaluates to true (and the conditional’s then-block is executed), the other for the case where b evaluates to false.

More reasoning rules will be introduced along the tutorial.

5 Example

In this tutorial, we use a banking example written in ABS to illustrate how specifications can be written and how programs are verified by KeY-ABS. The source code of the example are available here. The zip file contains the ABS banking module and two corresponding specification files.

The file Account.abs is the ABS implementation of the model, in which the deposit method increases the balance of the bank account by an input amount x, and the withdraw method decreases the balance by an input amountx if balance is at least x.

We specify an invariant for the AccountImpl class that “the balance is always positive”. Class invariants are specified in the .key file. Listing 2 gives a template of .key files. It contains the directory of the .abs file, the ABS module name, the ABS class, and a list of class invariants. In addition, user-defined functions, predicates, proof rules and proof-obligation formulae may also be included in the .key file.

The parameters of the \invariants section include a variable, historySV, of type Seq for recording the history of communication; a variable, heapSV, of type Heap for storing the state of fields; and a variable, self, of typeABSAnyInterface as the identity of the current class instance. Each class invariant has a name and is formulated in the following format:

Where the select function returns the value of the object field, i.e. balance, from the heap, i.e. heapSV, of the current class instance, i.e. self.

5.1 Verifying the AccountImpl class against the nonNegativeBalance Invariant

leftStart the KeY-ABS system (shown in Figure 2). Click the green folder icon and enter the directory of the unzipped example folder where the .abs and .key files are located. Select nonNegativeBalance.key file (see Figure 3). A Proof-Obligation Browser window, shown in Figure 4, will then pop up, by which we select the withdraw method of AccountImpl. Then the KeY-ABS proof obligation generator will generate an ABS Dynamic Logic (ABSDL) formula, shown in Figure 5.

Listing 4 presents the generated ABSDL formula for the withdraw method. It expresses that Forany calling object caller and any input data x_0 of type int, if the caller is not null, theinitial history is wellformed, the class invariant is satisfied before executing withdraw and theexecution of withdraw terminates, the class invariant must be proven upon the termination ofwithdraw.

Figure 3: Invariant selection

Figure 4: Proof obligation browser

Now click the green arrow icon. The automatic proof system of KeY-ABS will then be triggered. The proof obligation of the withdraw method can be automatically proved by KeY-ABS. This is shown in Figure 6, where a popped up window lists the number of nodes and branches in the closed proof tree recorded on the left side of the window.

Figure 5: Automated generated proof obligation

Figure 6: Closed proof and statics for method withdraw

5.2 Open goals of the proof tree

Now follow the same procedure and try to prove the class invariant nonNegativeBalance for the deposit method. You may notice that KeY-ABS cannot close the proof automatically. The open goal is shown in Figure 7. To find out the reason why the proof cannot be closed, we may check if the class invariant is too weak/too strong, or if additional proof rules are required, or simply the method implementation is not correct. If we look closely to the formula at the right side of the window, shown in Figure 7, while encountering the open goal, we notice that there is an assumption: x_0_0 <= –1. It expresses that the parameter x_0_0 of deposit method is not positive. The proof cannot be closed because the balance might not stay positive upon the termination of deposit when the input parameter data is negative. KeY-ABS symbolically generates two execution branches in the proof tree for deposit: one for positive method parameter and one for negative method parameter. In order to prove that the balance is still positive after executing deposit, we need to strengthen the class invariant by specifying an additional property, i.e. the methodparameter for deposit is always non-negative. This is captured by the history-based class invariant, amountOfDepositNonNegative, shown in Listing 5. History is a sequence of events recording the communication between objects. History based verification of ABS can be found in [4].

In Listing 5 the function seqLen(a) returns the length of the input sequence a. The function seqGet(a,b) returns the elements of the input sequence a at index b. The predicates isInvocationEv(ev) and isInvocationREv(ev) return true if the input event ev is an invocation event and invocation reaction event, respectively. These two events records the method invocation from the caller side and the starting of method execution at the callee side. The functiongetMethod(ev) returns the method name contained in the event ev. The function getArguments(ev) returns the sequence of method parameters contained in the event ev. Method names are expressed in the following format:

where a list of method parameters and the names of interface and class that the method belong to should be given. Class invariants are specified based on communication histories. This additional class invariants literally expresses that for all the invocation events and invocation reaction events for the deposit method, the method parameters contained in the events are always larger and equal to zero.

The updated specification is written in amountOfDepositNonNegative.key, shown in Listing 6, which contains two class invariants, nonNegativeBalance and amountOfDepositNonNegative. By selecting amountOfDepositNonNegative.key and the deposit method, KeY-ABS generates a proof obligation for deposit such that the conjunction of these two invariants should be proven upon method termination.

It has been proven that both withdraw and deposit methods preserve this strengthened class invariant. However, the proof is semi-automatic for both methods due to the need of quantifier instantiation. In the following section we prepare a step-by-step guidance for interacting with the KeY-ABS prover and accomplishing the proof.

5.3 Step-by-step interactive proof

Interactive proof can be non-trivial and requires certain user experience. Depending on the proof strategy, there are different ways to interact with the prover to close a proof. This section will show some basic tips to interact with KeY-ABS through an example. A screencast of how to prove the example is provided in the end of the chapter.Users can easily exploit the tool further after fully understand this tutorial.

To begin with we need to find where to prune the proof and how to continue the proof from there. To prune the proof means to select a node in the middle of the proof tree and then cut off the whole subtrees below this node. In KeY-ABS this is done by clicking a node in the proof tree and press the scissors icon in the top bar of the window. A straight forward place to prune the proof is the place in the proof tree right after KeY-ABS symbolically execute the whole method body. This state is captured by the node of the form < n >: { } in the proof tree. The symbol {} expresses that the method body is now empty and this reasoning rule removes the empty modality box in the ABSDL formula. Note that there are several nodes of the same form < n >: { }. We here choose the one closest to the root of the tree. We prune the proof right after the node 28: { }, because the current node will be pruned as well.

In Figure 8 we show the proof obligation formula generated after the execution of deposit method. Now we can clearly see that our goal is to prove the satisfaction of class invariant CInv(history,heap,this) based on the updates updated by the method execution. This updates captures the updated state of the field in the heap, where balance is increased by x_0_0. In addition, the initial history sequence has been extended with an invocation reactionevent and a completion event capturing the starting and the termination of the deposit method.

Figure 8: Prune the proof

Figure 9: One Step Simplification

The automated proof search implemented in KeY-ABS can be interleaved with interactive ruleapplication The KeY-ABS prover has a graphical user interface that is built upon the idea of direct manipulation. To apply a rule, the user first selects a focus of application by highlighting a (sub-)formula or a (sub-)term in the goal sequent. The prover then offers a choice of rules applicable at this focus. Rule schema variable instantiations are mostly inferred by matching. Accordingly, now we move the mouse on the formula until both the updates and the class invariant are highlighted. This step is shown in Figure 9. Then we left-click the mouse and select a reasoning rule calledOne Step Simplification. This reasoning rule substitutes the parameters of the CInv predicate with the symbolic values provided by the updates.Then we highlight the whole CInv predicate and apply the reasoning ruleinsertClassInvariantFor<Account.AccountImpl> over the predicate. This step is shown in Figure 10. This rule substitutes the history and the object field balance in the class invariants with the symbolic values contained in theCInv predicate. The substitution result is shown in Figure 11.

Figure 10: Insert class invariant.

Figure 11: Instantiated class invariant

Figure 12: Rule andRight

The class invariant is a conjunction of nonNegativeBalance and amountOfDepositNonNegative. Let’s prove them one by one. As the standard first-order-logic rule for conjunction at the right side of an implication will cause a split of proof tree into two branches, one for each conjunct,

the result of applying the rule andRight (Figure 12) on the class invariant can be found in Figure 13.

The first proof branch: The first case is to prove that the balance is non-negative upon method termination (shown in Figure 13). Since we have an assumption that the class invariant holds at the beginning of the method execution, we know that the method parameter of deposit is non-negative and the balance is non-negative before increasing the value. Based on these two assumptions, we should be able to close this proof branch. Detailed proof steps are described below. First we apply the rule insertClassInvariantFor<Account.AccountImpl> on the class invariant predicate CInv at the left side of the implication. The class invariant predicate CInv is written as the following:

This step is presented in Figure 14. Next we apply the standard first-order-logic rule, i.e., andLeft in KeY-ABS, on the class invariant.

This rule rewrites the conjunction of two formulae on the left side of an implication into a list of two formulae. This step is shown in Figure 15.

Figure 13: Branching in the proof tree

Figure 14: Insert class invariant at the left side of the implication

Figure 15: Rule andLeft

Now we would like to use the assumption of amountOfDepositNonNegative. Since there are quantifiers in amountOfDepositNonNegative invariant, we need to instantiate the quantified variables. There are two reasoning rules which can be applied for instantiating quantified variables. One is allLeft and another is allLeftHide.

The difference is that the former one keeps a copy of the highlighted universal quantified formula in addition to an instantiated version. This is the standard reasoning rule for first-order-logic while universal quantifier is at the left side of the implication. To keep the proof formula compact, we choose to apply the reasoning rule allLeftHide, which hides the copy of the original universal quantified formula. Note that by applying this rule a popup window will show up. This step is shown in Figure 16.

KeY-ABS pops up a window for users to type in a symbolic value that the quantified variable should be instantiated to. Users can also select an existing term from the formula, drag and drop it to the blank box in the window to instantiate the variable. But first we need to have an idea about what we can instantiate the variables to? The amountOfDepositNonNegative invariant contains two quantifiers: one for invocation events or invocation reaction events of the deposit method, and one for the index of the history sequence locating the event. That means we need to apply allLeftHide rule twice. As we know the history at the beginning of the method execution is the concatenation of initial history with an invocation reaction event of thedeposit method shown below:

and variable i to seqLen(history), which is the length of the history sequence. This index points out where the selected invocREv event locates. These instantiation steps are shown in Figure 17 and Figure 18.

Now we can close this proof branch by right-clicking the OPEN GOAL node and choosing Apply Strategy.

Figure 16: Rule allLeftHide

Figure 17: Instantiation of event variable ev

Figure 18: Instantiation of index variable i

The second proof branch: The second branch shown in Figure 19 is to prove that the method parameter of deposit is non-negative upon method termination. This can easily be proved because we have an assumption that the method parameter is non-negative at the beginning of method execution. To achieve this, we first apply the reasoning rule allRight on the class invariant at the right side of the implication.

This is a standard first-order-logic rule which instantiates universal quantified variables at the right side of an implication to fresh variables. Since there are two universal quantifiers, we need to apply allRight rule twice. This step is shown in Figure 20. The prover then automatically instantiates the quantified variable ev to a fresh variable ev_0 and variable i to a fresh variable i_0. The instantiated formula is shown in Figure 21. Note that the fresh variables created by the rule allRight are not necessarily ev_0 and i_0. It depends on how many proofs were done before. The subscripts could be different.

Figure 19: Proof of case 2

Figure 20: Rule allRight

Figure 21: Automatic instantiation

Class invariant CInv at the left side of the implication should be expanded, and the rules andLeft and allLeftHide are applied on the expanded class invariant. The same proof procedure can be found in the first proof branch and is shown in Figure 14 ~ Figure 16. There are two universal quantifiers. That means we need to apply allLeftHide rule twice. We manually instantiate the universal quantified variable ev to the fresh variable ev_0 and variable i to fresh variable i_0 at the left side of the implication. This step is shown in Figure 22 and Figure 23. Note that the subscripts of the automatically generated fresh variables could be different. The manual instantiation of the universal quantified variables at the left side of the implication here should always be consistent with the generated ones at the right side of the implication.

Figure 22: Instantiation of ev

Figure 23: Instantiation of i

Finally, we close the whole proof for the deposit method by clicking the green triangle button at the top-left corner of the window. The proof result is shown in Figure 24.

Figure 24: Close the proof for the deposit method

Screencast of the proof : At http://folk.uio.no/crystald/deposit.mov a screencast showing how to prove the deposit method can be downloaded. Now the readers can try to prove that withdraw method also preserves nonNegativeBalance and amountOfDepositNonNegative such that the AccountImpl class preserves all class invariants.

6 Conclusions and Further Reading

This tutorial presents the system workflow for the KeY-ABS theorem prover. It provides the links for readers to download the tool and example. This tutorial is self-contained such that the readers are able to reproduce the proof and obtain the basic knowledge of KeY-ABS. A simple sequential program, a banking account example, is used in this tutorial to teach the basic functionalities of KeY-ABS. For further reading, we recommend the following reading list. The KeY book [1] lays the foundation of KeY-ABS, specifically for the logic system, the symbolic execution engine and the taclets language for formulating reasoning rules. History-based reasoning for ABS is presented in [2]. A tool paper of KeY-ABS is in [3]. For more complex verification applications on unbounded concurrent ABS programs, one can read [4].